Method of and apparatus for locating envelope-tube damage at individual nuclear fuel elements in a reactor core



Dec. 31, 1968 R. HOLZER ETAL 3,419,467

METHOD OF AND APPARATUS FOR LOCATING ENvELoPEw'uEE DAMAGE AT INDIVIDUALNUCLEAR FUEL ELEMENTS IN A REACTOR coma Filed Oct. 24, 1965. Sheet of i5V A I 1b Fig. 1

Dec. 31, 1968 I R. HOLZER ETAL 3,419,467

METHOD OF AND APPARATUS FOR LOCATING ENVELOPE-TUBE DAMAGE AT INDIVIDUALNUCLEAR FUEL ELEMENTS IN A REACTOR CORE Filed Oct. 24, 1965 Sheet 2 of 5I v I D "'1l r fiI- I 1 (,1

I I I I I I I I I a l I I I f w I 01 N I I I l I I (PI l f| I l '1 I lIJI, I TI 5 \----Jl N l I II: 11 II: N I: H: [:3 C: I: .1

3 1968 R. HOLZER ETAL 3,

, METHOD OF AND APPARATUS FOR LQCATING ENVELOPE'TUBE DAMAGE ATINDIVIDUAL NUCLEAR FUEL ELEMENTS IN A REACTOR CORE Filed Oct. 24, 1965Sheet of 5 Fig.3

United States Patent 8 Claims. (51. 176-19) ABSTRACT OF THE DISCLOSUREMethod of locating envelope-tube damage of individual nuclear fuelelements in a reactor core includes coarsely checking respectivemulti-element regions of the core for occurrence of damage to thusdetect any suspicious core region, individually and sequentiallyremoving the fuel elements from the suspicious region and inserting theminto a pressure vessel for testing the respective fuel elements, rinsingeach element in the testing vessel with water and repeatedly changingpressure and temperature of the water in the testing vessel andmeasuring the concentration of fission products contained in the water.Apparatus for carrying out the above-mentioned method includes means forrepeatedly changing the pressure and temperature of the water in thetesting vessel.

Our invention relates to a method'of locating envelopetube damage atindividual nulear fuel elements in a reacfor core.

The methods heretofore known for this purpose required checking thereactivity of individual coolant channels or groups of such channelsduringreactor operation. This is possible because the occurrence ofdamage to fuel elements results in spontaneous issuance of volatilefissionproduced isotopes into the coolant of the reactor. However, thisphenomenon virtually ceases after shut-down of the reactor since only atthe high operating temperatures do the fission products have asufficient mobility to diffuse out of the nuclear fuel, particularly ifthe fuel is of the ceramic type. After shut-down, the discovery ofdamage to the fuel elements, that is as yet not optically discernible,is very diflicult, especially after removal of the particular elementsfrom the reactor core.

But even with nuclear reactors in which it is notpossible to superviseindividual fuel elements during operation, it is very difficult topinpoint damaged fuel elements that cause contamination of the coolantby radioactive fission products.

It is an object of our invention to solve these problems in a relativelysimple manner.

To this end, and in accordance with our invention, we proceed by firstchecking the reactor'core, portion by portion, that is, successively inrespective multi-element regions, as to occurrence of envelope-tubedamage. This first checking is performed in a course manner not yetsuitable to pinpoint individual elements, for example, by measuring thechanges in activity occurring in the coolant circulation system whenindividual regulating rods of the reactor are being moved.

After one of the checked regions has been found to be suspicious, thefuel elements are individually removed from this region and displaced toa testing device in the storage space or cavity of the reactor. Eachelement then found to be undamaged is returned back from the storagecavity to its seat in the core region, and each element now found to beaffected by envelope-tube damage is exchanged for a new fuel element.

3,419,467 Patented Dec. 31, 1968 In the testing device within thestorage tank or cavity of the reactor, the fuel elements areindividually subjected to Washing or rinsing by repeated changing ofwater pressure and temperature, and the water is then checked in aseparate vessel as to its content of fission products, especiallytellurium and substances of its disintegration series (iodine, xenon).Such testing devices are preferably mounted within the element storagespace of the reactor, because at this locality the necessary shieldingconditions are automatically met.

The method of the invention will be further described with reference tothe accompanying drawings, in which:

FIG. 1 shows schematically an example of a system with associated pump,filler device, filters and other accessories required for performing themethod of the invention;

FIG. 2 shows schematically, partly in section and somewhat exploded,components of another system for performing the method according to theinvention; and

FIG. 3 shows schematically a slightly modifiedform of the system of FIG.1.

The devices shown in FIG. 1 are designed to check the fuel elements bywashing suitable radioactive fission products out of the interior of adefective fuel rod and to then detect these fission products in thesurrounding water. Sometimes the readily volatile fission productsissuing during operation of the reactor, may already be lost when takingthe fuel element out of the reactor core and inserting the element intoa testing container. Nevertheless, there still remain fission productsin the interior of the fuel element, which have issued from thefissionable material and accumulated at the inner side of the tubularenvelope without having escaped to the outside. These accumulatedradioactive fission products disintegrate, so that the method accordingto the invention is effective to rinse them out of the envelope throughminute envelope fissures, if the tube exhibits such damage, thus makingis possible to sense or measure the fission products from the outside.Notable among such fission products is tellurium with its dissociationproducts iodine and xenon. Of course, the accumulation of the telluriumisotope in the interior of the fuel rods occurs not only at the innersurface of the envelope tube, but also within the expansion space of theenvelope structure, the holding springs and any other inserts with whichthe element may be equipped.

The equipment shown in FIG. 1 operates as follows:

The coolant circulating through the reactor core is being continuouslysupervised as to radioactivity. An increase in radioactivity isindicative of the fact that at least one fuel element has becomedefective and must be removed to prevent further contamination. Forfinding this particular element, its approximate locality must first bedetermined. For this purpose, individual regulating rods are displacedin the reactor core, one after the other, and any outbreak of fissionproducts resulting during each such rod displacement is being observed.In this manner the various multi-elernent regions or portions of thereactor core are checked. If thus a region is found to be suspicious ordefective, the fuel elements are individually taken out of this region,one after the other. This may be done, for example, with the aid of aconventional manipulating bridge and a fuel-element gripper. The removedindividual fuel element is then inserted into a testing vessel 1. Forshielding reasons, this vessel is located at the bottom of the fuelstorage tank or the storage cavity within the surrounding shieldingstructure of the reactor plant, usually located immediately beneath thereactor core. After insertion of the fuel element 17, the vessel 1 isclosed by a plug which, like the manipulator, is actuable from theoutside.

The vessel 1 has a double cylindrical wall In to form a jacket to betraversed by water for cooling the vessel. Connected in the water supplyline 1b is a cooling and filtering plant 8 which maintains the water inthe storage cavity of the reactor at a uniform temperature anddeactivates the water circulating from body of water below the level 10.through the filter 8 and the jacket 1a back into the surrounding water.

In the embodiment shown in FIG. 1, the interior of the testing vessel 1is connected through a water line 1c with a measuring vessel 4 outsideof the water contained in the storage cavity. The water content of thetesting vessel 1 can be pumped into the measuring vessel 4 to maintain acirculation of the water quantity between the vessels 4 and 1 until anexchange of the respective water volumes has taken place. Valves 6, 7and 9 afford controlling this circulation which is maintained by meansof a pump 2, also securing the required pressure increase within thetesting vessel 1. The pump 2, in conjunction with a water-jet pump 3also affords obtaining a negative pressure in the testing vessel 1.Inserted into the measuring vessel 4 is a suitable measuring device 15such as a scintillation counter.

For testing the fuel element 17 in vessel 1, the valve is first closed,thus stopping the flow of coolant through the jacket space 1a. Now thetemperature in the interior of testing vessel 1 increases since theafter-disintegration heat of the fuel element 17 is no longer dissipatedby a flow of coolant. Thereafter the valves 6, 7 and 9 are closed andthe pump 2 is switched on. This places the testing vessel 1 underpositive pressure. After a predetermined period of time, the valves 9and 10 are opened so that the jacket space 1a of the testing vessel 1 isagain cooled by a flow of water. The water-jet pump 3 is also placed inoperation, thus producing a negative pressure in testing vessel 1.

If previously, on account of any leakage, water inclusions have beenformed in the gap between the nuclear fuel proper and the envelope tubeof the element 17, the application of negative pressure causes suchwater to rapidly evaporate and to subsequently expand into thesurrounding water. Thereupon the valve 9 is again closed and the valves6 and 7 are opened. The pump 2 now constrainedly transfers the watervolume from testing vessel 1 into the measuring vessel 4. Then the pump2 is switched off, and the activity in vessel 4 is measured with the aidof the scintillation counter 15.

Thereafter the cooling of the testing vessel 1 is again discontinued sothat renewed heating of this vessel occurs, and simultaneously thetesting vessel 1 is placed under positive pressure. This is followed byagain passing a flow of coolant through the jacket space 1a and applyingnegative pressure with the aid of the water-jet pump 3, whereupon thevalves are switched in the above-described manner to pass the content oftesting vessel 1 to the measuring vessel 4. The activity is againmeasured, and the described process is repeated several times.

If the fuel element 17 in testing vessel 1 is defective, an increase inactivity is noted after each step of circulation. When thus a damagedfuel element is recognized, this element is taken out of the testingvessel and lowered onto the bottom of the storage cavity, whereupon anew fuel element is substituted in the reactor core at the locality ofthe one previously removed and tested. If no damage has been found toexist at the fuel element tested, the same element is withdrawn from thetesting vessel 1 and immediately returned into the reactor core.

The activity Washed out of the defective fuel element 17 with the aid ofthe above-described device, is subsequently rinsed into the storagecavity from which it is eliminated through the appertaining circulatorysystem and in the filter equipment 8 by an ion exchanger or otherdevices known and used for such purposes.

Now the device is available for testing another fuel element, and theabove-described operations are performed in the same sequence. It ispreferable to control these operations by a program-switching or timingmechanism so that the measured values always remain comparable with eachother.

The method according to the invention may be performed by a variety ofother devices, for example those exemplified in FIG. 2. According tothis illustration, the fuel elements are taken from the reactor core 25with the aid of an element gripper 27, this being indicated by an arrow24. The gripper is then shifted along the manipulating bridge 26, andthe withdrawn fuel element is inserted into a tubular conveyor system12, this being indicated by an arrow 34. The conveyor passes the fuelelements 17 to the bottom of the storage cavity or vessel 20 here shownto form the bottom portion of a tank. Remotely controlled slide gates 22and 23 permit dividing from the conveyor system 12 a chamber 21 whichcorresponds to the testing vessel 1 in FIG. 1, and which, accordingly,is in communication with a measuring vessel 4 and a water-jet pump 3,also with a pressure pump 2 and the water lines and valves, shown inFIG. 1 but for simplicity omitted in FIG. 2.

After testing the fuel element 17 in chamber 21, the gates 22 and 23 areopened and the fuel element is passed along in the tubular conveyor atwhose other end it is again received by the element gripper 27 to beeither set down into the storage tank or cavity or be transported backinto the reactor core 25.

Which particular construction of the device for performing the method ofthe invention is to be chosen, depends upon the size and shape of thefuel elements to be checked, upon the amount of storage space available,and also upon the space available between the reactor core and thestorage space.

It will be understood that the component devices, connecting conduits,valves and other auxiliaries shown in FIGS. 1 and 2 are represented onlyto the extent they are necessary and essential to the method accordingto the invention. As a rule, additional devices for measuringtemperature, overpressure and safety valves, as well as devices forsecuring a constant water level in the measuring vessel 4, are required.However, since such auxiliaries throughout are conventional, it appearsunnecessary to discuss them further in this specification.

Relative to the scintillation counter 15, it may be added that it isdesirable to mount it in the testing vessel in such a manner that anoptimal yield of the radiation quantums reaching the crystal of thecounter is obtained, this being advisable on account of the small andrelatively uncertain value of radioactivity in the water. As a rule,therefore, it is preferable to mount the scintillator at the center ofthe testing vessel, and to give this vessel and approximately sphericalshape. With such an arrangement, the indicated gamma-flux may beapproximately twice as large as if the counter is mounted at the edge ofthe vessel. Connected to the scintillation counter, if it is located atthe center point of the measuring vessel, is an optical guide or lightconductor 151 which extends through the wall of the testing vessel andis connected with a secondary-electron multiplier 152. This has theadvantage that the photo-cathode of the electron multiplier may be keptat normal room temperature, thus preventing a temperature-dependentincrease in thermal electron emission which essentially constituted thezero effect.

According to another way of performing the method of the invention, therinsed-out radioactivity is collected in the illustrated circulation ofthe rinsing liquid and is concentrated in this circulation system, forexample, by insertion of an ion exchanger. This affords considerablyincreasing the detection probability at very slight liberated.

quantities of radioactivity. It will be understood, of course, that therinsing circulation need not be operated with water but may also employother fluids, for example, gases. This is sometimes particularlyadvantageous for fuel elelents used in a gas-cooled nuclear reactor. Thecorresponding equipment for thus applying the invention is shown in FIG.3 and is analogous to that described above with regard to FIGS. 1 and 2,except that the rinsing gas is not released into the water cavity ortank but is returned into a gas-storage container 30. The concentrationof the radioactivities issued into the rinsing medium can then bedetermined, for example, with the aid of a cooling trap 31 which issupplied with coolant through the conduits 32.

In view of the heat of subsequent disintegration generated in the fuelelements 17 being tested, and also on account of the gage pressure ofabout 10 atmospheres impressed by means of the pump 2, the watertemperature in the testing vessel may rise up to about 180 C. During thetesting period in which the water-jet pump and the cooling of testingvessel 1 are active, the gage pressure may be reduced to approximately/3 atmospheres. It is not at all detrimental if superficial boiling ofthe liquid takes place on the fuel element itself. Consequently,overheating of the fuel can be reliably prevented, since care is takenfor good heat dissipation outside of the testing vessel 4.

Of course, when testing fuel elements exhibiting slight d liberation ofsubsequent disintegration heat, a separate heating of the surroundingwater may be applied, for example, up to 250 C. During the subsequentpressure reduction, the water penetrated into the enveloping tube of thefuel element evaporates out, so that a good heat exchange is secured.

If desired, the testing method according to the invention may beperformed continuously by providing for a sufiiciently small testingvessel 1 and consequently also for sufficiently short temperature andpressure pulsations at the element being tested. Particularly in thecase of such a continuous performance, it is advantageous to promote theremoval of the radioactive fission products by application of sonic orultrasonic vibrations.

The testing method according to the invention is not limited to nuclearreactors previously shut down, but may also be employed with allreactors that afford an exchange of fuel elements during operation. Itis not limited to measuring or sensing the radiation issuing fromtellurium, but may also be performed with the aid of sensors or gagesresponding to other fission products, depending upon the particularfissionable material of the fuel rods.

We claim:

1. Method of locating envelope-tube damage of individual nuclear fuelelements in a reactor core, which comprises coarsely checking respectivemulti-element regions of the core for occurrence of damage to thusdetect any suspicious core region, individually and sequentiallyremoving the fuel elements from said suspicious region and insertingthem into a pressure vessel for testing the respective fuel elements,rinsing each element in the testing vessel with water and repeatedlychanging pressure and temperature of the water in said testing vessel,and measuring the concentration of fission products contained in thewater.

2. The method according to claim 1, which comprises moving individualregulator rods in said core and simultaneously measuring the resultingchange in radioactivity within the coolant circulation of the reactor tothereby effect said coarse checking of said respective core regions.

3. The method according to claim 1, which comprises constrainedlyrecycling the rinsing water between the testing vessel and a measuringvessel, and effecting said measuring of said concentration within saidmeasuring vessel.

4. Method according to claim 1, including measuring the content oftellurium and its disintegration products in the rinsing water when thereactor is shut off.

5. With a nuclear reactor having a core with fuel ele ments and havingan element storage space which contains water, the combination ofapparatus for locating envelope-tube damage of individual nuclear fuelelements, said apparatus comprising a pressure vessel located outside ofthe reactor core and sequentially accommodating individual fuel elementstransferable thereto from a suspicious region of the reactor core fortesting said fuel elements in said pressure vessel, a measuring vessel,a water circulatory system including said testing vessel and saidmeasuring vessel and having circulation control means for transferringwater from said testing vessel to said measuring vessel, means forrinsing each element in said testing vessel with the water, means forrepeatedly changing the pressure and temperature of the water in saidtesting vessel, and radiation-responsive gage means in said measuringvessel for measuring the activity of the water transferred from saidtesting vessel as indicative of envelope-tube damage.

6. Apparatus according to claim 5, comprising a tubular conveyor deviceextending from said core to said storage space and having gate means fordividing an underwater chamber from said conveyor device, said chamberforming said testing vessel.

7. An apparatus according to claim 5, said measuring vessel and saidcirculatory system being mounted below the water level of said storagespace.

8. Apparatus according to claim 5, wherein said testing vessel islocated outside of said storage space.

References Cited UNITED STATES PATENTS 2,846,872 8/1958 McAdams et al7345.5 3,073,767 1/1963 Whitham et al 176-l9 X 3,142,625 7/1964 Wellborn176-32 FOREIGN PATENTS 1,328,935 4/1963 France.

OTHER REFERENCES Kelly: Burst Slug Detection in Water-Cooled ReactorsJuly, 1959, of Nuclear Power, pp. 77-79.

Osborne: Locating Failed Fuel in Water Reactors, July 1961, ofNucleonics, pp. 84, 86, 89.

REUBEN EPSTEIN, Primary Examiner.

US. Cl. X.R. 23-45 .5

